Panels of gypsum board having a core of set gypsum reinforced or faced on the outer surfaces thereof by sheets of facing material have long been used in the fabrication of buildings. For example, such members are commonly used as components in fabricating partitions or walls for rooms, elevator shafts, stairwells, ceilings, roof decks and the like. Although paper sheets have long been used as the reinforcing material for gypsum board of this type, facing materials formed of fibrous mat, such as a glass fiber mat, have enjoyed a substantial increase in popularity. This popularity is due in large part to the technologies described in the following U.S. Patents: U.S. Pat. No. 4,647,496; U.S. Pat. No. 4,810,569; U.S. Pat. No. 4,849,173; U.S. Pat. No. 5,148,645; U.S. Pat. No. 5,220,762; U.S. Pat. No. 5,319,900; U.S. Pat. No. 5,342,680; U.S. Pat. No. 5,371,989 and U.S. Pat. No. 5,397,631, each of which has been assigned to the assignee of the present invention and incorporated herein by reference. These patents disclose, generally, gypsum board reinforced on at least one surface thereof with a fibrous mat comprising glass fibers bound together by an adhesive binder.
Gypsum board products of the type disclosed in the above patents have been commercialized and are presently being used effectively in an increasing number of applications. Such applications include those in which conventional paper-faced gypsum board is considered unsuitable for use or in which the use of fibrous mat-faced gypsum board has been recognized to provide an advantage.
Gypsum board is generally manufactured by a method which includes dispersing a gypsum slurry onto a moving sheet of a fibrous mat. The mat is typically supported by equipment such as forming tables, support belts, carrier rolls and/or the like. A second sheet of fibrous mat is then fed from a roll onto the top of the slurry, thereby sandwiching the slurry between two moving fibrous mats. Forming means, such as forming or shaping rolls, are utilized to compress the slurry to the desired thickness. The gypsum slurry is allowed to at least partially set and then sequential lengths of board are cut and further processed by exposure to heat, which accelerates the drying of the board by increasing the rate of evaporation of excess water from the gypsum slurry.
Because the board is dried after the fibrous mats have been placed thereon, it is known that the mats should be sufficiently porous to permit the excess water to evaporate at an acceptable rate. On the other hand, excessive penetration of the slurry through the mat and onto the underlying support surfaces and equipment are highly undesirable. Accordingly, certain prior art techniques have utilized a fibrous mat joined to a release-paper backing to help prevent soiling of support equipment by the slurry which would otherwise leak through the mat. This release-paper is typically removed and discarded after processing is complete. Utilization of such a mat, however, adds unwanted cost and complexity to the manufacturing process and is therefore undesirable.
A highly successful and effective method for minimizing soiling of support equipment by the gypsum slurry is disclosed in U.S. Pat. No. 4,810,569. This patent discloses that excessive slurry penetration can be substantially avoided by adjusting the viscosity of the slurry so that penetration of the slurry into the mat is controlled. The recommended technique for controlling the viscosity of the slurry is to add thereto a viscosity-control agent, such as paper fiber, cellulosic thickeners, bentonite clays and starches.
Although the viscosity-control techniques identified in the '569 patent are valuable and widely used for the purpose of preventing excessive slurry bleed-through, applicant has found that in certain circumstances such techniques are less than fully effective and/or undesirable for other reasons. For example, adjusting the content of certain viscosity-control agents, such as paper fibers, can have an undesirable or unwanted impact on the physical or other properties of the resulting board. In addition, applicant has found that in certain circumstances viscosity-control techniques, when used alone, can not control slurry penetration to the extent desired. For example, applicant has found that difficulties with the control of slurry bleed-through occur when viscosity-control techniques are utilized alone to form relatively thin gypsum board products and/or when relatively high compression force techniques are used to shape and form the slurry.